Many prior art wheeled vehicles use a rotating drive shaft to transmit power from the engine to the wheels. For instance, the drive shaft may transmit power from the engine location to a differential spaced between two wheels, with the differential transmitting power from the drive shaft outward to half shafts driving each of the two wheels. In the manufacture of such vehicles, the engine, the drive shaft and the differential are separate components which are assembled together into the frame of the vehicle.
It may be desired for space and layout purposes to center the engine at a different transverse or vertical location than the differential, i.e., the axis of rotation of the engine output may not be linearly aligned with the axis of rotation of the differential input but instead may have a vertical, horizontal and/or angular offset. Different vehicles in the same line of vehicles may have different longitudinal spacing between the engine and the differential, requiring different lengths of drive shafts. Further, due to manufacturing and assembly tolerances, the offset spacing may not always be exactly identical from vehicle to vehicle on the assembly line, or the longitudinal length required of the drive shaft may not be exactly the same from vehicle to vehicle on the assembly line.
One mechanism that is commonly used to allow for different vertical, horizontal and/or angular offsets is a universal joint in the drive shaft as shown in FIG. 1. Each end of the drive shaft 1 includes a universal joint 2 made up of a cross-cardan joint 3 connected to a cross-cardan fork 4. Each cross-cardan joint 3 includes a flange 5 which can be bolted, shown bolted on the left to the terminal pad 6 on the engine and shown bolted on the right to the terminal pad 7 of the differential or other mount for the drive axle. Because the universal joint 2 can transmit torque across different bend angles, the universal joint 2 allows considerable flexibility in the vertical, horizontal and/or angular offset, both as designed and as assembled. In the prior art embodiment shown in FIG. 1, minor adjustments in length of the drive shaft 1 can be taken up by a mid-shaft spline connection 8. Alternatively, one of the cross-cardan joints 3 could have a splined connection (not shown) to either the engine output 6 or the differential input 7, to accommodate a variation in the length of the drive shaft 1.
While such drive shafts allow considerable flexibility, they have various disadvantages. Prior art drive shafts are commonly heavy in weight, decreasing the fuel economy and handling of the vehicle. The flange 5 for the bolted end connection has a large radius of rotation, requiring sufficient clearance to install the bolts 9, which may interfere with other vehicle layout objectives. Prior art drive shafts may have strong dynamic unbalance and induce strong vibrations in the operation of the vehicle, increasing wear on various vehicle components, resulting both in increased noise and reduced service life. The prior art drive shaft structures may have high manufacturing costs, particularly since assembly, maintenance and changing of the universal joints 2 is more complex. In addition, when prior art drive shafts are roughcast with different lengths, multiple molds may be needed and the mold cost is increased. Better solutions are needed.